1. Field of the Invention
The present invention relates to the art of earth boring with rolling cutter drill bits. In particular, this invention relates to an improved thrust bearing for sealed and lubricated three cone earth boring bits utilized for gas and oil well drilling.
2. Description of Related Art
Sealed and lubricated rolling cutter drill bits (also called rock bits) typically have three different bearing structures in each cutter. The first bearing structure is designed to handle cantilevered radial loads and is typically a journal bearing or a roller bearing.
The second bearing structure is designed to retain the rolling cutter upon the cantilevered bearing spindle when the cutter is subjected to offward thrust. This retention system is generally comprised of either ball bearings or a friction bearing such as a snap ring or a threaded retaining ring. The third bearing structure is designed to carry onward axial thrust loads and is most often a friction type bearing. This thrust bearing in rolling cutter drill bits is the object of the present invention.
Analysis of used rolling cutter drilling bits shows that when high loads are combined with high rpm, the thrust bearing often fails or the resulting heat build up causes degradation of the other bearings. Even though a great many designs and materials for rock bit thrust bearing have been used in an attempt to solve this problem, thrust bearing performance still remains a source of bearing failure, especially at very high rpm.
A number of bearing material and lubrication schemes have been used in the past by drill bit designers to improve thrust face performance. Lubricant circulating systems, as shown in U.S. Pat. Nos. 3,841,422; 3,844,364; 4,167,219; 4,181,185; 4,183,416; 4,240,674; 4,390,072; 4,412,590; 4,446,933; 4,452,323; 4,501,338; and 5,099,932 promote the flow of fresh lubricant through the bearings with minimal pressurization.
Means of pressuring lubricant in a rock bit to prolong bearing life are shown in U.S. Pat. Nos. 2,906,504; 3,244,459; and 3,866,695. In these designs the lubricant in the entire bearing cavity and around the bearing seal is pressurized. This pressurization can severely limit the life of the bearing seal, however, because seal life depends, in part, upon how long the seal is subjected to a given pressure differential.
Typically, a rock bit bearing seal will survive for no more than a few hours with a constant 300 psi pressure differential. If the prior art bearing pressurization systems were applied to modern drill bits, the bearing seals would have to withstand differential pressures in excess of 1000 psi for long periods of time, perhaps more than 100 hours, and could experience peak differential pressures greater than 5000 psi.
Many of the above patented lubrication systems take advantage of the reciprocating piston action of the rolling cutter upon the bearing spindle of a drill bit to provide the pumping action. Drill bits typically have 0.010"-0.025" axial play of the cutter upon the bearing spindle. As described in U.S. Pat. No. 3,137,508, lubrication flow as the rolling cutter moves axially along the bearing spindle can cause pressure fluctuations in the lubricant of up to 1800 times per minute during operation. In many of the above lubricant pressurization and circulating patents, this flow has been harnessed to provide power for the lubricant pumping system.
Another scheme to improve thrust bearing performance in rock bits is a hydrodynamic lubrication system at the thrust bearing face, such as shown in U.S. Pat. Nos. 5,188,462 and 5,265,964. Hydrodynamic schemes are intended to increase the lubricant film thickness at the thrust face interface. Although the film thickness can increase slightly in these designs, the thrust bearing still operates in a thin film, boundary layer lubrication regime, and the thrust bearing life does not appear to significantly improve.
Finally, a great number of bearing material, tribological, and lubricant engineering systems are known in the art, and are intended to increase the bearing life and/or reduce the rubbing friction at the thrust face under the typical thin film, boundary layer lubrication present at the thrust faces of typical modem sealed and lubricated rolling cutter drill bits.
The present invention provides a new thrust face bearing/lubrication system which utilizes pressurized lubricant at the thrust face area to provide a thick, hydrostatic lubricant film. This thick, hydrostatic lubricant film helps to prevent asperity contact of the mating thrust bearing surfaces, reducing friction and wear, and thus prolonging bit life. The thick, pressurized film is maintained by capitalizing upon the normal piston effect of the cutter upon the bearing spindle to pump lubricant through a one way valve into the thrust area. The passageways for the lubricant to flow out of the thrust area are deliberately restricted, allowing the lubricant to become pressurized. The restrictions in the lubricant flow out of the thrust area are placed so that only the thrust face area and adjacent bearings are pressurized, leaving the bearing seal in the drill bit to operate conventionally, without unusually high pressure differentials.
As the cutter moves on and off the bearing spindle during operation, the thrust faces will separate by the amount of axial play allowed by the cutter retaining bearing, forming a clearance gap. In a typical rolling cutter drill bit, the clearance gap (and consequentially the lubricant film thickness) will be 0.010" to 0.025". When the cutter experiences an onward axial thrust load, the lubricant becomes pressurized, carrying the load. The lubricant will flow around the restriction and out of the thrust area at a rate related to the onward load. As the lubricant flows out, the clearance gap is reduced.
After a time, the onward load will reverse, the cutter will again be pushed off the bearing spindle, and the cycle will repeat.
As long as the cycle time of the onward/offward cutter loading is shorter than the time required for the lubricant to bleed from the thrust face area (at the applied load) the thrust faces will not contact.
It is therefore the object of this invention to provide a sealed and lubricated rolling cutter drill bit with an improved lubricant system which provides selectively pressurized hydrostatic thrust bearing lubrication by allowing lubricant to flow into the thrust bearing area easily and restricting the flow of the lubricant out of the area without pressurizing the bearing seal.